1. Field of the Invention
The present invention relates to a conductive reflective film, which is formed by coating a composition containing metal nanoparticles using a wet coating method and calcining the resultant, has high reflectance when measured from the substrate side, and exhibits a low specific resistance as much as that of a bulk, and a method for producing the same.
2. Description of the Related Art
Research and development in the field of clean energy technologies are currently promoted from the environmental protection standpoint. Solar cells in particular are drawing attention since their energy source is sunlight, which is infinite and free from environmental pollution. Conventionally, a bulk solar cell, which employs a semiconductor prepared by slicing a bulk crystal such as monocrystalline silicon and polycrystalline silicon into a thick plate, has been used for photovoltaic power generation. However, the above silicon crystals used in the bulk solar cell require a large amount of energy and time for crystal growth as well as a complex step in the subsequent production process, thereby making it difficult to increase production efficiency and to provide solar cells at low cost.
On the other hand, in a so-called thin film semiconductor solar cell (hereinafter referred to as a thin film solar cell), in which a semiconductor layer made of amorphous silicon or the like having a thickness of a few micrometers or less is used as a photoelectric conversion layer, semiconductor layers which are to become photoelectric conversion layers may be formed as many as required on a low cost substrate such as glass or stainless steel. Accordingly, it is expected that this thin film solar cell will become a major type of solar cell in the future for its reduced size and weight, low production cost, and easy modification in terms of area increase.
The thin film solar cell is classified into a super straight type and sub straight type based on its configuration. The super straight type solar cell in which light enters from the transparent substrate side usually has a structure where the substrate, a transparent electrode, a photoelectric conversion layer, and a back surface electrode are formed in this order. In the super straight type solar cell in which the photoelectric conversion layer is formed of a silicon-based material, there has been a study in order to enhance its power generation efficiency by producing the cell with a structure where, for example, a transparent electrode, an amorphous silicon, a polycrystalline silicon, and a back surface electrode are formed in this order (for example, refer to Shozo Yanagida et al “Frontier developments of thin film solar cells—towards higher efficiency, large scale production, and widespread promotion” published by NTS Corporation, March, 2005, pp. 113, FIG. 1 (a) (“Yanagida”). In the structure described in Yanagida, the amorphous silicon and the polycrystalline silicon form a photoelectric conversion layer.
When the photoelectric conversion layer in a solar cell is configured from a silicon-based material, the photoelectric conversion layer will have a relatively low absorption coefficient due to the above material. Accordingly, when the photoelectric conversion layer has a film thickness on the order of a few micrometers, part of the incident light will transmit the photoelectric conversion layer, and this transmitted light does not contribute to power generation. For this reason, the power generation efficiency is generally improved by reflecting the light, which is not absorbed and which transmits the photoelectric conversion layer, with a reflective film in order to return the light back to the photoelectric conversion layer by either making the back surface electrode function as a reflective film or forming a reflective film on the back surface electrode.
Electrodes and reflective films have been formed by the vacuum film forming methods such as the sputtering method in the development of thin film solar cells to date. However, considerable cost was generally required for the maintenance and operation of a large film forming apparatus. Hence, use of the wet film forming methods instead of the vacuum film forming methods for forming these electrodes and reflective films has been studied, and it is expected that the switching to the wet film forming methods will considerably improve the running cost of the apparatus.
Examples of the conductive reflective film formed by a wet film forming method include the reflective film disclosed in, for example, Japanese Unexamined Patent Application, First Publication No. Hei 5-95127 (sections [0015], [0020] and [0021] on the detailed description of the invention) (“JP '127”) which is formed in the back surface side of a photoelectric conversion element using the electroless plating method. With the method shown in JP '127, the document describes possible improvements in productivity by forming the reflective film using the electroless plating method. Specifically, in the above method, a resist film which will be a protective film for the plating is first formed in the front surface side of a substrate by overall printing, and then a pretreatment is conducted in the back surface side of the substrate using a solution, in which HF is added in a proportion of 2 to 4% by mass to the pretreatment solution for a non-conductor. Thereafter, a reflective layer composed of a copper plating film having a thickness of about 3 μm is formed using an electroless plating solution. Then this substrate is subjected to an ultrasonic cleaning process in a solvent to remove the resist film, thereby forming a photoelectric conversion element.
However, in the electroless plating method shown in the above JP '127, the protective film for the plating is first formed in the front surface side of the substrate followed by the pretreatment of the side, which will be subjected to a plating treatment, using the HF solution, and thereafter a process for immersing the substrate is conducted. Accordingly, generation of waste liquid in addition to the complicated production process is expected to become a problem.
As a simpler method, a method is disclosed where a solution, in which ultrafine metal particles are dispersed in an organic solvent, is coated and followed by a sintering process at a low temperature of 100 to 250° C. (for example, refer to Japanese Unexamined Patent Application, First Publication No, Hei 9-246577 (section [0035] on the detailed description of the invention) (“JP '577”)). With the method shown in the above JP '577, without employing a high vacuum process, it is possible to form a metal electrode having high reflectance as well as high conductivity and which has a large area while being uniform.
However, the metal film obtained by the method shown in the above JP '577 tends to have lower reflectance in the substrate side compared to the reflectance in the side of an exposed surface, which is situated in the opposite side. This is because pores are usually generated between a metal film and a substrate, on which the film is formed, when forming the metal film by coating the dispersion liquid containing ultrafine metal particles onto the substrate and calcining the resultant. When pores are generated between the metal film and the substrate, it is assumed that the light entering inside the pores is ultimately attenuated due to the repeated reflection inside the pores. In addition, the reflected light that reached the substrate side is also assumed to be attenuated when its angle of incidence with respect to the substrate surface is large. This may be due to the increase in the proportion of light, which is totally reflected at the interface between a medium with a low refractive index (i.e., the air inside the pores) and a medium with a high refractive index (i.e., the substrate), and the attenuation occurring in accordance with the proportion.
As an example of the method to form a metal coating film exhibiting high reflectance and plating-like metallic luster on the substrate surface, a method is disclosed in which a metal thin film is formed by coating a coating material containing metal colloidal particles onto a substrate, drying the resulting coating film, and thereafter heating this coating film to fuse the colloidal particles in this coating film (for example, refer to Japanese Unexamined Patent Application, First Publication No. 2000-239853 (sections [0015], [0097] and [0098] on the detailed description of the invention) (“JP '853”)). However, the reflectance on the substrate side is not considered even with the method shown in JP '853.